pthreadpool: enable shared libraries (#24657)

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>

var/spack/repos/builtin/packages/pthreadpool/package.py

@@ -50,6 +50,7 @@ class Pthreadpool(CMakePackage):
 
     def cmake_args(self):
         return [
+            self.define('BUILD_SHARED_LIBS', True),
             self.define('FXDIV_SOURCE_DIR',
                         join_path(self.stage.source_path, 'deps', 'fxdiv')),
             self.define('GOOGLETEST_SOURCE_DIR',